From 9fddc2160c524d4fefb9fc8a42704f812aef7bf3 Mon Sep 17 00:00:00 2001
From: "simonpj@microsoft.com" <unknown>
Date: Fri, 7 Dec 2007 17:05:07 +0000
Subject: [PATCH] Remove debug warning, and explain why

---
 compiler/main/TidyPgm.lhs     |   10 +++++++---
 compiler/stranal/WorkWrap.lhs |   25 ++++++++++++++-----------
 2 files changed, 21 insertions(+), 14 deletions(-)

diff --git a/compiler/main/TidyPgm.lhs b/compiler/main/TidyPgm.lhs
index 1f65d21..131b617 100644
--- a/compiler/main/TidyPgm.lhs
+++ b/compiler/main/TidyPgm.lhs
@@ -817,7 +817,7 @@ tidyWorker _tidy_env _show_unfold NoWorker
   = NoWorker
 tidyWorker tidy_env show_unfold (HasWorker work_id wrap_arity) 
   | show_unfold = HasWorker (tidyVarOcc tidy_env work_id) wrap_arity
-  | otherwise   = WARN( True, ppr work_id ) NoWorker
+  | otherwise   = NoWorker
     -- NB: do *not* expose the worker if show_unfold is off,
     --     because that means this thing is a loop breaker or
     --     marked NOINLINE or something like that
@@ -825,8 +825,12 @@ tidyWorker tidy_env show_unfold (HasWorker work_id wrap_arity)
     -- then you can make the simplifier go into an infinite loop, because
     -- in effect the unfolding is exposed.  See Trac #1709
     -- 
-    -- Mind you, it probably should not be w/w'd in the first place; 
-    -- hence the WARN
+    -- You might think that if show_unfold is False, then the thing should
+    -- not be w/w'd in the first place.  But a legitimate reason is this:
+    -- 	  the function returns bottom
+    -- In this case, show_unfold will be false (we don't expose unfoldings
+    -- for bottoming functions), but we might still have a worker/wrapper
+    -- split (see Note [Worker-wrapper for bottoming functions] in WorkWrap.lhs
 \end{code}
 
 %************************************************************************
diff --git a/compiler/stranal/WorkWrap.lhs b/compiler/stranal/WorkWrap.lhs
index fbc0bf7..3667da8 100644
--- a/compiler/stranal/WorkWrap.lhs
+++ b/compiler/stranal/WorkWrap.lhs
@@ -376,17 +376,7 @@ worthSplittingFun ds res
   = any worth_it ds || returnsCPR res
 	-- worthSplitting returns False for an empty list of demands,
 	-- and hence do_strict_ww is False if arity is zero and there is no CPR
-
-	-- We used not to split if the result is bottom.
-	-- [Justification:  there's no efficiency to be gained.]
-	-- But it's sometimes bad not to make a wrapper.  Consider
-	--	fw = \x# -> let x = I# x# in case e of
-	--					p1 -> error_fn x
-	--					p2 -> error_fn x
-	--					p3 -> the real stuff
-	-- The re-boxing code won't go away unless error_fn gets a wrapper too.
-	-- [We don't do reboxing now, but in general it's better to pass 
-	--  an unboxed thing to f, and have it reboxed in the error cases....]
+  -- See Note [Worker-wrapper for bottoming functions]
   where
     worth_it Abs	      = True	-- Absent arg
     worth_it (Eval (Prod ds)) = True	-- Product arg to evaluate
@@ -403,6 +393,19 @@ worthSplittingThunk maybe_dmd res
     worth_it other	   	     = False
 \end{code}
 
+Note [Worker-wrapper for bottoming functions]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We used not to split if the result is bottom.
+[Justification:  there's no efficiency to be gained.]
+
+But it's sometimes bad not to make a wrapper.  Consider
+	fw = \x# -> let x = I# x# in case e of
+					p1 -> error_fn x
+					p2 -> error_fn x
+					p3 -> the real stuff
+The re-boxing code won't go away unless error_fn gets a wrapper too.
+[We don't do reboxing now, but in general it's better to pass an
+unboxed thing to f, and have it reboxed in the error cases....]
 
 
 %************************************************************************
-- 
1.7.10.4